Steering systems for construction vehicles include those provided with steering hydraulic cylinders as actuators for performing steering of a vehicle, a pilot-operated directional control valve for controlling a direction and amount of pressure oil to be fed to the steering hydraulic cylinders, and a steering-wheel-operated signal generation means provided with a rotationally-operable steering wheel and having a rotary directional control valve or the like for generating a pilot signal such that by rotational operation of the steering wheel, a pilot signal is generated to switch an operated position and opening area of the pilot-operated directional control valve. Examples of construction vehicles equipped with such a steering system include articulated construction vehicles that upon turning, their self-propelled bodies are articulated to make them go in a curve, and as their representative examples, articulated wheel loaders can be mentioned.
In such an articulated construction vehicle, a self-propelled body is divided into a front frame supported by front wheels and a rear frame supported by rear wheels, and both the frames are connected rotatably relative to one another in a horizontal direction such that the self-propelled body can be articulated. Also provided as hydraulic actuators for articulating the self-propelled body are body-articulating hydraulic cylinders selectively extendable or retractable such that the front frame can be driven to rotate relative to the rear frame in the horizontal direction. The steering systems for construction vehicles include those equipped with steering hydraulic cylinders, typified by such body-articulating hydraulic cylinders, as vehicle-steering actuators. Steering systems for construction vehicles, said steering systems being equipped with such actuators, can be divided roughly into those of the wheel type that they are provided with a steering wheel to be rotationally operated and those of the lever type that they are provided with a lever to be tilt (pivot)-operated.
It obviously becomes necessary to make the articulated construction vehicle travel when performing work, for example, when performing digging work or loading work of excavated earth and/or sand on a truck by a wheel loader at a construction site. Because such a wheel loader can travel at 35 km or so per hour, it would be convenient if the wheel loader can be designed to permit traveling to go and come on general roadways in addition to traveling to perform such work. In fact, an articulated construction vehicle is allowed under regulations to travel on general roadways. When traveling on general roadways, however, it is required under regulations to operate a steering system through a steering wheel. For this reason, wheel-type steering systems have heretofore been generally adopted for construction vehicles such as articulated construction vehicles.
A wheel-type, construction-vehicle steering system provided with hydraulic cylinders as hydraulic actuators for steering as mentioned above is disclosed, for example, in Patent Document 1. Considering the features of such a wheel-type, construction-vehicle steering system, operation of a steering wheel is rotational operation as opposed to tilt operation of a lever, and the steering wheel is extremely large in the amount of maximum operation compared with the lever. This construction-vehicle steering system can, therefore, perform fine control. Concerning the operation of a steering system, traveling on a general roadway requires to frequently perform fine control such as making the vehicle turn subtly although it is unnecessary to frequently perform such control as making the vehicle go in a large curve. A construction vehicle provided with a wheel-type steering system is hence suited for traveling on general roadways. The construction vehicle provided with such a wheel-type steering system can be driven or operated with the same maneuvering feeling as general cars, and in this respect, is also suited for traveling on general roadways.
Patent Document 1: Japanese Utility Model Application No. SHO 63-49087 (JP-B-63-49087 U) (pages 1-3, FIG. 4)